(1) Field of the Invention
The present invention relates to an imaging ellipsometer based on a polarization modulation method.
(2) Description of the Related Art
Various techniques for determining the physical properties and film thickness of thin films, such as thin films composed of inorganic compounds, metals, and like inorganic materials; thin films composed of biological substances, organic compounds, and like organic substances; etc., have been developed and utilized in various technical fields.
Ellipsometry is a technique that makes it possible to determine the optical properties (e.g., refractive index) and film thickness of thin films by using a light beam to irradiate a thin film formed on a flat substrate. In addition to being able to measure in a vacuum or in air, ellipsometry enables measurement to be conducted in a variety of media, such as water, an organic solvent, and the like by the use of a dedicated container. Moreover, ellipsometry can be used for measuring under various conditions. For example, ellipsometry allows measurements over a wide range of light spectra, from ultraviolet/visible light to infrared light, using, as a light source, various lamps, such as a laser beam light, a halogen lamp light, and a xenon lamp light in addition to a laser beam light. Therefore, ellipsometry can be used to measure both inorganic and organic substances, and is thus employed in various industrial and technical fields.
Ellipsometers are classified into reflection and transmission types depending on the disposition of the sample to be measured. The former type analyzes light that has been reflected from the sample from the measurement beams directed at the sample, and the latter type analyzes transmitted light. Hereinafter, reflection ellipsometers will be explained in detail, but it is a matter of course that the description also applies to transmission ellipsometers.
In measurements using ellipsometry, the polarization state of the incident light is generally divided into a p component (which vibrates in parallel with the plane of incidence), and an s component (which vibrates perpendicular to the plane of incidence). The polarization of incident light changes before and after it is reflected from the surface of a film sample, depending on the properties of the film sample. The change in polarization is expressed by the two ellipsometric parameters of Ψ shown in formula (1) and Δ shown in formula (2):Ψ=tan−1(|rp|/|rs|)  (1)Δ=δrp−δrs  (2)In these formulae, |rp| represents the absolute value of the reflectance of a p component; |rs| represents the absolute value of the reflectance of an s component; δrp represents a change in the phase of a p component; and δrs represents a change in the phase of an s component. To be specific, Ψ represents a change in light intensity occurring when incident light reflects from the surface of a sample, and, similarly, Δ represents a change in phase. The film thickness and refractive index of a sample can be determined from the parameters obtained by these measurements using a calibration curve obtained by fitting, theoretical transformation, experimentation, or simulation.
The overall arrangement of an ellipsometer is classified into a PCSA, PSCA, or other types. In a PCSA ellipsometer, a first polarizer P and a compensator C (¼ wave plate) are disposed along the incidence path of an incident-light optical unit, and a second polarizer A (hereinafter referred to as an analyzer), and a photomultiplier tube, a photodiode, or other means for detecting emitted light are arranged along the emission path of an emitted-light optical unit. The first polarizer P transmits only the polarization component that has a fixed angle (hereinafter referred to as an azimuth) relative to a predetermined axis of coordinates, and converts randomly polarized light into specific, linearly polarized light; the compensator C converts the linearly polarized light into elliptically polarized light by delaying the phase of one polarized light by 90° relative to another polarized light whose azimuth is perpendicular to the phase of said one polarized light; and the second polarizer A transmits only the polarization component that has a specific azimuth among the light that has been reflected from a sample S. In a PSCA ellipsometer, the compensator C, the analyzer A, and the detector are arranged along the emission path of an emitted-light optical unit.
As shown in the above examples, an ordinary ellipsometer has an incident-light optical unit that irradiates a sample with a luminous flux having a predetermined cross section, and an emitted-light optical unit having a means for detecting an emitted light, such as a photomultiplier tube, photodiode, or the like. Therefore, the optical properties and film thickness of a sample measured using such an ellipsometer are the average optical properties and average film thickness of the sample for the area upon which the luminous flux was irradiated.
However, the film sample to be measured generally has a fixed surface area, and the optical properties and film thickness generally differ from place to place along the surface of the sample. Thus, depending on the measurement purpose, it may be more important to obtain the accurate optical properties and film thickness of the sample for each of the measured points and their distributions, rather than the average optical properties and film thickness of the entire sample. For example, in manufacturing semiconductor processes, since a fine thin-film pattern is formed onto a silicon wafer using photolithography, it is important to determine the composition and thickness of each of several points of the pattern at each step. Recently, in order to manufacture various electronic devices, optical devices, etc., the formation of two-dimensional crystals composed of an organic compound on a solid surface, such as a metal, semiconductor, or derivative, has been studied. This makes it important to determine the optical properties and thickness in two dimensions. Moreover, in order to quickly identify the genes and proteins involved in bio-phenomena, such as for preventing the development of cancer, widespread attention is presently being focused on a technique for efficiently detecting the DNA and proteins contained in samples using a microarray in which various kinds of DNA and antibodies are arranged on a flat substrate. The DNA and proteins contained in a sample are adsorbed on the substrate by specific interactions with the DNA and proteins, respectively, that are fixed on the substrate. A technique for determining, in two dimensions, the DNA and proteins that adsorb on the substrate is indispensable.
There are several reports on two-dimensional measurement using an ellipsometer. For example, according to an imaging ellipsometer with a rotary analyzer disclosed in Patent Document 1, the azimuths of the polarizer and compensator are adjusted to a given angle, the analyzer is rotated, and the reflected light is measured with a CCD camera at the four azimuths of 0°, 45°, −45°, and 90°, after which ρ and Δ are arithmetically calculated from the azimuths of the polarizer, analyzer, and compensator.
According to an imaging ellipsometer disclosed in Non-Patent Document 1, the phase difference δ of a p component relative to an s component of an incident light is changed at given intervals by adjusting the azimuths of a polarizer and compensator to a given angle and rotating a compensator, and the intensity of reflected light is measured with a CCD camera at each phase difference δ, after which Ψ and Δ (hereinafter referred to as ellipsometric parameters) are arithmetically calculated from the obtained intensity values of the reflected light.
However, in these devices, the measurement precision is inferior to ordinary measurements using a single detector, often by an order of magnitude or more, and the measurement takes a long time.
In particular, as compared with the polarization modulation ellipsometer, which allows highly precise measurement, the difference in precision can exceed three or more orders of magnitude. In a polarization modulation ellipsometer, ellipsometric parameters are determined by periodically changing the polarization state of the light used for measuring at frequencies in the range of several tens of kHz to several hundreds of kHz, and performing frequency analysis of the changes in light intensity over time that are measured by a detector. The polarization modulation ellipsometer also features a high S/N ratio in the obtained results. This is because only a specific frequency component of a light signal can be selectively detected, and optical elements are not mechanically driven. In addition, the time required for obtaining ellipsometric parameters is as short as several milliseconds and noise can be easily decreased by extending the measuring time. In the initially announced device, a specific frequency component was detected using a lock-in amplifier (Non-Patent Document 2). A method for performing Fourier transformation of a signal that was subjected to high-speed A/D (analog-to-digital) conversion was then proposed (Non-Patent Document 3). With these devices, ellipsometric parameters Ψ and Δ can generally be measured at a precision of 10−3 degree and 10−2 degree, respectively, and when the measuring time is set to about 10 seconds, the precision can be further improved by about 10−2.
In contrast, since the CCD detection rate reaches video rates (about several tens of Hz to several hundreds of Hz) at most, a CCD cannot detect light signals modulated at several tens of kHz to several hundreds of kHz when the detection rate is maintained in its original condition. Thus, a method for achieving lock-in detection similar to that when using a single detector was developed by periodically changing the intensity of the measuring beam that is directed at the sample at the same fundamental frequency as a phase modulator (Non-Patent Document 4). For example, when a measuring beam is periodically blocked to be converted into a continuously pulsed light, the light signal is detected only while the measuring beam is incident on the sample. This makes it possible to perform frequency analysis of phenomena with high change rates by using minute sensors, each corresponding to a pixel, in a CCD imaging sensor. By basing the ON/OFF duration on the cycle of the drive signal of a phase modulator, the direct-current component of an output signal, a component having the same phase as that of the drive signal of the phase modulator (a sine wave component), and a component having a phase difference of 90° (a cosine wave component) can be extracted. In addition, specific harmonic components can be extracted by modulating the amplitude of the measuring beam using a signal that includes a sine wave and a finite number of harmonic components, thus utilizing the nature of a trigonometric function.
The parallel synchronous detection that is attained by modulating the amplitude of the measuring beam has been applied to light interference microscopes and the like to thereby improve their precision and increase their functions (Non-Patent Documents 5 and 6).
A method for measuring the ellipticity and polarization direction of light in real time was devised by developing a time-correlated image sensor in which the CCD gate is controlled by an external signal (Non-Patent Documents 7, 8 and 9).
[Patent Document 1] U.S. Pat. No. 5,076,696
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